It is likely that key steps in the pathogenesis of syphilis are mediated by interactions between the host and the surface of Treponema pallidum. We have learned that the cell envelope structure of T. pallidum differs from that of gram negative bacteria in fundamental ways relevant to identification of virulence related outer membrane components. Virulent T. pallidum exhibits relative resistance to binding of specific antibodies to its surface. The association of its outer membrane with the treponemal cytoplasmic cylinder is fragile. We have found that the non-ionic detergent Triton X-114 (TX-114) solubilizes the outer membrane, but does not disrupt the periplasmic endoflagella or the cytoplasmic membrane, as evidenced by retention of the penicillin binding proteins we have defined. Electron microscopy of thin sections has corroborated the solubilization of the outer, but not cytoplasmic membrane by TX-114. The major polypeptide antigens released by TX-114 partition into the TX-114 phase at temperatures above the cloud point (20 degrees) of this detergent, as expected for hydrophobic membrane proteins. The TX-114 released outer membrane antigens include the 4D and 38kDa proteins we have described, the 47kDa protein under study in several laboratories, and polypeptides of 34, 29, 20, 16 and 14kDa. These native T. pallidum proteins are available in the milligram amounts needed for experimental biology. We hope to learn the individual and collective significance of these proteins to killing of T. pallidum in vitro and in vivo and to attachment of T. pallidum to host cells. We have already learned that the 4D surface antigen can modify the course of experimental syphilis in immunized rabbits in a manner consistent with partial protection, and that 4D antibodies can block T. pallidum attachment to human cells. We have also found that the endoflagella of T. pallidum are the target of in vitro treponemicidal antibodies. This periplasmic organelle will be studied in conjunction with the outer membrane proteins of the organism.